This invention relates generally to a system for discriminating a radiation-contaminated material and an apparatus for measuring the radioactivity of such material, and more particularly to a system for discriminating radiation-contaminated, fragmented material and an apparatus suited for measuring the radioactivity of such fragmented material.
In reactor facilities, a large amount of fragmented or broken waste (e.g., concrete waste) contaminated by radiation to a very low level is produced when the reactor is discarded. Therefore, it is necessary to accurately measure the radioactivity of such fragmented waste and also to discriminate the fragmented waste in a short time.
One example of a fragmented material discriminating system, as well as an apparatus for measuring the radioactivity of the fragmented material, is disclosed in FIGS. 3 and 4 of Japan Atomic Energy Association Journal (Vol. 29, No. 11 (1987), Page 60). In this conventional example, fragmented concrete waste, produced as a result of breaking a building structure, is passed through a radiation detector by a belt conveyor extending through the radiation detector. At this time, the radioactivity of the fragmented waste is detected. The radioactive concentration of the waste fragments is expediently estimated from the counting rate of the radiation detector and the empirically-obtained density of the fragments. Based on the radioactive concentration thus expediently estimated, the waste fragments are discriminated at the distal end of the belt conveyor.
In the measurement of radioactivity, the background count is proportional to the volume of the detector. In the measurement of radioactivity of a very low level, .sup.40 K contained in the detector is also a major factor for the background count, and its influence is proportional to the volume of the detector.
The lower limit value D (.mu.ci/g) of the detection of the radioactivity is represented by the formula, D=K.multidot.Ns, where K ((.mu.ci/g)/cps) represents the radioactive concentration conversion factor, and Ns represents the limit counting rate. The radioactive concentration conversion factor depends on the reciprocal of the measurement efficiency (including the absolute efficiency and geometrical efficiency of the detector) of the measurement system. If the count after a time t is Nm, and the background count is nb, then the net count N can be represented by (Nm-nb). (N.apprxeq.nb) is obtained around the detection limit value, and therefore the standard deviation .sigma..sub.t is represented by the following formula: ##EQU1##
If the limit count is 3.sigma..sub.t, then its limit counting Ns is 3.sqroot.2nb/t. Therefore, the detection lower limit value D is K.multidot.3.sqroot.2nb/t.
Even if the object to be measured is so arranged as to be completely surrounded by the detector, the geometrical efficiency will not exceed the maximum of 100%. It will be appreciated from this that when the measurement time t of the measurement system is kept constant, the decrease in the background count nb is an important point for the low-level measurement. In other words, the detection lower limit value becomes smaller in proportion to the one-half power of the background counting rate.
The measurement precision is influenced by the density of the fragmented material (object to be measured).
Therefore, in order to accurately measure the radioactivity of the fragmented material, it is necessary to know the density of the fragmented material in the radiation detector.
In the above prior art, since the radiation detector surrounds the belt conveyor, its volume is large. Therefore, the background counting rate is large, and the detection lower limit value is large. Therefore, the radioactivity of a very low level can not be measured. In other words, the measurement sensitivity is low, and hence the discrimination of the fragments can not be carried out in a short time. Further, when the radioactive concentration is to be determined from the radiation count, the empirically-obtained density is used, and therefore the results of the measurement of the radioactive concentration merely serve as expedient ones.